New study finds apex fossils aren't life:
Cyanobacteria evolved at least 2.4 billion years ago, but their morphology resembles the much more ancient structures found in the 3.5-billion-year-old Apex Chert. Credit: NASA
Structures thought of as the oldest known fossils of microbes might actually be microscopic mineral formations not associated with life, suggesting that astrobiologists have to be careful calling alien objects “life” when scientists have trouble telling what is or was alive on Earth.
More than 20 years ago, microscopic structures uncovered in the roughly 3.5-billion-year-old Apex Chert formation in western Australia were described as the oldest microbial fossils. These structures were interpreted as cyanobacteria, once known as blue-green algae, embedded in a silica-loaded rock formed in a shallow marine setting. These structures were all detected in slices of rock just 300 microns thick, or roughly three times the diameter of a human hair.
However, the interpretation of the structures has always been controversial, and it is still hotly debated among scientists searching for Earth’s earliest evidence for life. Specimens from the site apparently displayed branching structures that some researchers said were inconsistent with life, while others dismissed such branching as artifacts from photo software. Analysis of the structures themselves suggested they were carbon-based, and therefore associated with the organic chemistry of life, but some contended they were a type of carbon known as graphite, while others said they were kerogen, a mixture of organic compounds.
Now University of Kansas geospectroscopist Craig Marshall and his colleagues have taken another look at the Apex Chert structures and determined they might not be carbon-based after all. Instead, they seem to just be a series of fractures filled with crystals.
"It's one of those funny moments in science when you go out to do one thing and it completely flips 180 on you," Marshall said.
The scientists collected 130 pounds (60 kilograms) of samples from the original site and made very thin slices 30 to 300 microns thick.
"We were interested in developing new methods of looking at ancient microfossils, and so we were drawn to the Apex Chert as these putative microfossils are so iconic," Marshall explained. "However, when we started working on the rocks, we discovered things were a little more complex than we thought they would be."
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Was this what Earth looked like during the Archean eon? Credit: Peter Sawyer / Smithsonian Institution
In the thicker slices, they saw reddish-brown features resembling the previously described microfossils. However, in the thinner slices, these structures appeared to be less like microbes and more like fractures.
These cracks seem to be filled with a light mineral possessing a coarse block-like texture, as well as with a dark mineral that came in thin plates. Further analysis suggests the lighter material was quartz and the darker matter was iron-rich hematite.
Provided by PhysOrg.com
